Fault currents or stray currents in complex power networks are hardly completely avoided. They can occur as stray currents, for example at electronic filters or as fault currents due to insulation deficiencies of the electronic equipments. Systems using grounded power networks in which the neutral point or neutral conductor are grounded have a particularly high risk of producing fault currents. A connection between the network and ground is usually provided close to the power source in order to keep the resistance between the network and ground small. The fault current will then return to the power source through the potential equalization system. As all current conduction parts of a facility have to be included in the potential equalization system, it is unavoidable that fault or stray currents flow through these conducting components. In accordance with the specific construction of the facilities, fault or stray currents may also flow through bearings or other mechanical devices thereby causing damages in these stray current sensitive mechanical parts. Another risk resides in the electrochemical corrosion of structure parts caused by the stray current. One possibility to avoid damages in mechanical parts consists in implementing the power network as an isolated IT-network. However, IT-networks have the disadvantage that first order insulation errors usually remain undiscovered, as no fault currents occur. In case of a first insulation error, the ground potential is equal to the fault voltage and dangerous situations can occur. Only a second insulation error might trigger the protective devices, as for example a fuse. In order to detect a first insulation error, insulation error detection systems have to be implemented which are expensive and complex.
A second possibility consists in implementing the network as a locally limited network. This provides that fault currents can only occur within a limited area. Fault currents are still not completely eliminated, but currents through sensitive mechanical components can be avoided through an adequate layout or arrangement of the facility. However, this requires implementing subsystems. The power source is then arranged close to the subsystem. If the power source is then supplied by a separate grounded network, fault currents can still occur.
A third possibility consists in providing specific current paths parallel to the mechanical parts. This is usually done by sliding contacts. However, the amount of fault current that may flow through the sliding contacts instead of the mechanical parts depends on the ratio of the impedances. Studies have shown that the impedance of mechanical parts is very often low enough to let fault currents still flow through the mechanical parts.
One attempt to avoid stray currents through mechanical parts in a wind power plant is known from WO 2007/107158 A1. This solution consists in placing two ground connections (one of which only provides an AC current path) on both sides of the driving shaft of the wind power plant.